Contactless communication portable devices such as contactless IC (integrated circuit) cards (also known as chip cards or smart cards), electronic labels or tags, or badges, operate on the basis of a communication by an electromagnetic field with a read and/or write interrogating device, generically referred to as a reader. Such contactless devices generally comprise a microcircuit connected to a parallel LC type resonant circuit. The inductor is an external antenna, while the capacitor is integrated to the microcircuit. The two form what is commonly known as a “tuned circuit”.
As an example, in some contactless IC card applications, the reader transmits an electromagnetic signal having a carrier frequency of 13.56 MHz.
This transmitted signal serves on the one hand to power the contactless card, which thus derives by induction the energy required for its operation, and on the other hand to set up a communication between the card and the reader according to an established protocol. Thus, when the contactless card penetrates into the transmission field of the reader, it communicates with the latter by a modulation operation which consists in modifying at least one parameter of the carrier.
The contactless device receives, via its tuned circuit, an amplitude modulated signal from the reader. The contactless device interprets the message from the reader by a demodulation operation which consists in extracting the modulated signal from the carrier. The frequency of the modulated signal is much smaller than that of the carrier, in general around ten kHz.
The quality and reliability of the RF communication are directly linked, among things, to the distance between the reader and the contactless device. The distance, or range, of the RF communication between the reader and the contactless device depends on several parameters including the tuning frequency between the resonant circuit of the contactless device and the transmission frequency of the reader, as well as the quality of demodulation of the modulated signal.
The quality of demodulation of the modulated signal depends directly on the distance between the contactless device and the reader, as well as the speed of displacement of the device in the transmission field of the reader. The greater is the range, and the more the device is stealthy, the more the demodulation shall be subject to error.
FIG. 1 is a block diagram of the input stages of the contactless device. A resonant circuit, centered at the carrier frequency, receives a modulated electromagnetic signal. A rectifier bridge generates a dc voltage in order to power the contactless device. The output voltage Vdb of the rectifier bridge represents the dc voltage after rectification and contains both the energy needed for self-powering the contactless device and the information of the modulated signal.
For applications that use 100% amplitude modulation, a diode isolates the resonant circuit from the load and thus eliminates all possibility of a return current to the resonant circuit. A limiter allows to maintain the power supply voltage Vdd below a threshold, e.g. of 4V. A resistor is advantageously placed between Vdb and the diode so as to isolate the modulated signal on Vdb. Thus, the demodulation of the modulated signal coming from the reader is performed directly from the signal Vdb at the output of contactless device's rectifier bridge.
FIG. 2 is a schematic illustration of a classical amplitude demodulation device.
The signal Vdb is first of all processed by an RC type low-pass filter so as to eliminate the components of the carrier and extract therefrom on the one hand the envelope of the modulated signal, generally referred to as the reference modulating signal Vmod, and on the other its continuous component DC. A cutoff frequency of a few tens of kHz can be chosen for that first filter. The continuous level DC is then extracted by another low-pass filter whose cutoff frequency is less than the frequency of the modulating signal Vmod, e.g. a few kHz. The demodulation signal Vdemod can then be obtained by comparison between the modulating reference signal Vmod and its continuous level DC.
Such a demodulation device presents a number of limitations due to its very structure. Indeed, the continuous level DC varies greatly as a function of the position of the card in the field of the-reader, and of its displacement speed. This situation makes it difficult to generate a reliable comparison level on a permanent basis. This problem is even more acute in applications where a large range, about 50 cm to 1 m, is required.
The graphs of FIGS. 3a to 3c illustrate the limits of classical demodulation devices.
The reference modulating signal Vmod is represented on FIG. 3a with its continuous level DC. The demodulation signal Vdemod is shown in FIG. 3b. It is observed that certain modulations can fail to be detected and that the modulations identified by the device are not perfectly identified, i.e. the start and end of modulation are not identified with precision.
FIG. 3c illustrates to this end the aim of the present invention, which consists in producing, for a contactless device, a demodulator capable of identifying with precision the start and end of all the modulated signals sent by the reader.